Blowout preventers are normally opened and closed by hydraulic opening and closing lines. However, when a blowout preventer is closed around the drilling string, the well bore pressure acts in the annulus about the drilling string to keep the preventer closed. As the well pressure increases, the net closing force also increases. When the drilling string which includes enlarged joints and drill collars is moved across the blowout preventer sealing element, which is called "stripping", it creates wear on the sealing element. With the closing forces, due to both the control system "closed" pressure and the well bore pressure, compressing the sealing element against the drill string the sealing element wears out at a rapid rate during stripping operations.
The present invention is directed to a method and apparatus in which the closing force caused by the annulus or well bore pressure is reduced at the discretion of the operator and the compensation can be desired percentage of the well bore generated closing force. The compensation force is applied to the blowout preventer in an opening direction sufficient to compensate for the closing forces generated by the well pressure for reducing the wear on the blowout preventer sealing elements but without opening the blowout preventer.